1. Field of the Invention
This invention relates to electrochemical storage devices such as batteries, and in particular to such devices which contain strong oxidizing agents as electrolytes, and more particularly to an improvement in the cationic separator employed in zinc-ferricyanide batteries and the like.
2. Description of the Prior Art
Membranes both of the cationic and anionic type, have found use in electrochemical storage devices to separate the electrolytes. In many applications, such as the zincferricyanide battery, the particular membrane utilized is required to exhibit a high degree of chemical stability, due to the high reactivity of the electrolytes. In the zincferricyanide battery, the catholyte is a strongly alkaline mixture of sodium ferri- and ferrocyanide. Alkaline ferricyanide is a moderately strong oxidizing agent which is capable of oxidizing a variety of organic compounds, such as phenols and alkyl-substituted aromatics, such as disclosed by B. S. Thyagarajan, Chem. Revs., 58, p. 439 (1958), and E. Muller et al., Ber., 93, p. 2649 (1960).
The prior art is replete with a wide variety of membranes for batteries, for example:
U.S. Pat. No. 4,157,423 discloses a separator which contains a preponderance of tertiary hydrogens, and thus is highly susceptible to oxidative attack of the ferricyanide.
U.S. Pat. No. 4,251,605 discloses acrylonitrile polymers that contain sulfonic acid groups which are of relatively unsatisfactory stability in the presence of oxidizing agents and strong alkali solutions.
U.S. Patent No. 3,998,871 discloses sulfonated isocyanate compositions which also have a relatively unsatisfactory resistance to oxidation.
U.S. Pat. No. 3,944,435; 4,260,874; and 4,049,866 may be of interest, but the batteries described in the patents are all based on electrolytes that are not capable of oxidation, while only the electrodes are electrochemically active. The pores of the separators utilized are filled with a single electrolyte. In contrast, in the present invention it is desired to obtain an improved membrane which would be resistant to corrosive environments, i.e., oxidizing and/or strongly alkali solutions. Other patents which may be considered pertinent are U.S. Pat. No. 3,709,841 and 3,855,122, but these patents relate to desalination, not devices for the storage of electrical energy such as batteries.
In any case, the art has been unable to develop a membrane which possesses adequate stability under oxidative environments such as the zinc ferricyanide battery. Currently, the only cationic membrane which exhibits sufficient stability is made from Nafion.RTM., a perfluorosulfonate developed by Du Pont, which is extremely expensive, and thus unsuitable for applications such as batteries which are utilized for utility load leveling applications, and the like.